1. Field of the Invention
This invention relates to the elimination of air pollution due to sulfur dioxide in industrial gases emitted into the atmosphere. Sulfur dioxide is scrubbed out of these gases by means of a reactive solution including potassium carbonate; sulfur is then recovered from the solution as hydrogen sulfide in a process which also recovers the reactive solution. The hydrogen sulfide product derived in the process may then be fed to a conventional oxidation process for the production of elemental sulfur.
2. Brief Description of the Prior Art
Sulfur is one of the most widely distributed of the elements. It occurs as a sulfide or as a sulfate in ores of metals; it is combined with organic and inorganic elements in fossil fuels. Hence in the extraction of metals from ores or in the combustion of fuels sulfur dioxide is often one of the products of the process and appears in the waste gases normally vented to the atmosphere. Although the amount of sulfur dioxide released into the atmosphere by any one source is relatively small the total quantity from all sources is large. For example, the total sulfur dioxide in the combustion products from electric power generating plants in the United States is over 20 million tons annually, substantially enough to supply all of the sulfur needed to satisfy the sulfuric acid market. Because power plant stacks are usually near densely populated areas, the effect of sulfur dioxide emissions on people and materials is within ready observation by a large segment of the population, and the result is a strong demand to abate sulfur dioxide pollution.
The widespread interest in the solution of this problem is clear when one considers the large number of processes which have been or are in various stages of development for the sole purpose of eliminating this air pollutant. Work on some of these was started even before the public clamor for action. The fact that not a single totally satisfactory method has been developed yet is a good indication of the difficulties which must be overcome to solve the problem.
Over 30 different processes have been proposed or are in various stages of development or use. These may be grouped into a few categories and may be classified generally as employing a wet or dry process.
Wet processes have had most of the attention, partly because at least a part of the technology needed was already available. These include scrubbing the gases with a lime, limestone, or dolomite slurry and discarding the spent slurry. A variation of this throw-away process is to scrub the gases with an alkali absorbent solution, and to regenerate the alkali by reaction with lime. The spent lime is discarded.
Other wet processes employ sodium or ammonium-based absorbent solutions. The ammonium-based solutions are not recoverable. One sodium-based plant is in limited commercial operation and delivers sulfur dioxide. Others are in various stages of development and will produce sulfuric acid or elemental sulfur.
Processes still in early stages of development include a formate process using a potassium salt, a citrate process, and a process using an undisclosed organic solvent.
Dry processes are generally in early development stages and include the use of activated carbon, or molten salts as the active sulfur dioxide removal agent. (See A. V. Slack, "Removing SO.sub.2 from Stack Gases" Environmental Science and Technology. 7 No. 2. 100-119 (1973) for a good summary of present status.)
Up to the present time, limestone or dolomite suspensions in water are the most widely used or in advanced pilot stages. There are about 25 full-sized plant installations and 25 pilot installations. The first application of this type operation was made in England about 40 years ago. Many of the design factors developed then appear to be substantiated in today's applications. The advantages of the process are that it has had commercial operation and that the raw material is relatively cheap. The disadvantages are that the spent absorbent cannot be recovered but must be discarded, that no saleable product is made to recover at least part of the cost of SO.sub.2 removal, and that, if it should be the only process used, the disposal of the spent chemical which itself may be a polluter could pose serious problems. Some work is being done to develop a method for recovering the absorbent for reuse in the process. This development, if successful, would eliminate the disposal problem and would make the process more attractive. Many operating problems, such as plugging in the absorber, are still encountered, however. Apparently the process has been installed to satisfy pollution abatement requirements primarily because it was the only available one which had been in commercial use previously.
The other wet processes are of limited interest at this time, because of cost or because much development work is still required or because the sulfur product of the process may have a limited market.
The ammonium and sodium-based absorbents, which employ regeneration by reacting spent solution with lime, avoid some of the absorption problems present in lime slurry scrubbing, but in recovering the reactive absorbent, calcium sulfite is produced just as in the slurry system and is confronted with the same supply and disposal problems.
Systems using ammonia solutions without regeneration have three important problems. Up to the present, a satisfactory way has not been found to prevent ammonia gas from passing into the atmosphere. In the second place ammonia cannot at present be readily recovered and recycled so the cost of ammonia becomes a significant charge against the operation. Finally the present product of the operation is an ammonium compound, such as ammonium sulfate, which has a limited market as a fertilizer.
Systems using sodium-based solutions without regeneration have the same features as the ammonia process except there is no problem due to fumes. The present product is sodium sulfite or bisulfite, which can be used in pulp mills, but this is a limited market and cannot be counted upon to take more than a little of the total possible production.
Sodium-based solutions with recovery and recycle are of interest because they have been extensively investigated since the early thirties. This process uses a sulfite/bisulfite cycle whereby the sulfur dioxide absorbed in the scrubber to form the bisulfite compound is released as a concentrated SO.sub.2 gas in sulfite recovery step. Sulfur dioxide can subsequently be converted to sulfuric acid or to elemental sulfur. The production of elemental sulfur is an important advantage because the sulfur originally present in the waste gas as sulfur dioxide is now in a non-polluting, non-corrosive state having greatest convenience in storage, transport, and use. One important economic factor is the cost of the heat required in the recovery step, and the cost of reducing sulfur dioxide to elemental sulfur. Technologically, processes for recovering sulfur from sulfur dioxide are not as well developed as those producing sulfur from hydrogen sulfide.
At one time the use of potassium sulfite instead of sodium sulfite was advocated because of its greater solubility in water than sodium sulfite and the cost of evaporating water in the recovery step would have been considerably less than for the sodium salt. In the development of the process, two unforeseen but important difficulties were encountered. In the absorber, potassium metabisulfite was formed in addition to potassium bisulfite when sulfur dioxide in the gas reacted with potassium sulfite in the solution. The metabisulfite which is the least soluble of the three compounds crystallized out of solution and plugged the tower. In addition to this problem, a much larger amount of potassium sulfate than predicted was produced in the recovery step. A market for this product was dubious and the cost of replacing potassium too high for the process and the use of potassium as an absorption medium was abandoned.
All of the wet processes in use or being developed have their advantages and disadvantages. None of them stands out clearly as the probable candidate for long-term applications.
Among the dry processes, limestone injection into the combustion chamber has been tried but is now abandoned because of operating difficulties and because it has failed to remove SO.sub.2 quantitatively. The activated carbon and the molten salt processes are still in early development stages.
Among the other processes mentioned only the molten salt process needs to be discussed because certain aspects of the process of this invention are somewhat similar. In the molten salt process, an eutectic mixture of lithium, potassium, and sodium carbonates in the molten state is sprayed into the hot waste gases where it reacts with sulfur dioxide to convert the metal carbonate into the metal sulfite. The carbonate is regenerated in a two-step process which releases a concentrated stream of hydrogen sulfide. Proven economical processes are available for converting hydrogen sulfide into elemental sulfur. The molten salt process, which is now in a pilot plant stage, is interesting because sulfur may be produced in its advantageous elemental form and because the active chemicals can be recovered and reused, thus eliminating the disposal problem.
There is, however, an important disadvantage arising from the fact that the flue gas must be contacted hot. Except for possibly a very few installations, the fly ash removal facilities of existing power plant steam generators are the last elements that the flue gas traverses in its passage from combustion chamber to stack. The gas temperature in this zone may be in the range of 250.degree.F. to 300.degree.F., a temperature which is well below the melting point of the eutectic. The molten salts cannot be sprayed into the flue gas ahead of the dust removal facilities where higher temperatures prevail, however, because the ash they would pick up would cause problems. Hence this process requires the removal of dust from hot gases. The use of hot electrical precipitators is possible but they have not yet been thoroughly proven in power plant service, and furthermore, are more expensive than electrical precipitators located at the cold end of the gas passage. Nevertheless, the process could be designed into new installations.
In many, if not in most of existing installations, its use would probably be difficult if not prohibitive, for three reasons. In the first place, the installation of the process would probably require the substitution of hot electrical precipitators in place of existing cold precipitators, an obviously expensive undertaking; in the second place, the setting of the existing plant usually does not have room or provision for making the kind of changes needed to install the hot electric precipitator and spray chamber. Finally, it is obvious that the unit being altered would be out of service during the construction period. The resultant loss in generating capacity during such period would be not only costly but also inconvenient in these times of energy shortages.
______________________________________ References on Prior Art Employing Molten Salt Processes U.S. Pat. Nos. ______________________________________ 3,438,722 L. A. Heredy, et. al. 3,438,727 L. A. Heredy 3,438,728 Le Roy F. Grantham 3,438,733 Le Roy F. Grantham, et. al. 3,438,734 Le Roy F. Grantham, et. al. 3,551,108 Le Roy F. Grantham ______________________________________
Progress Reports, Atomics International "Development of a Molten Carbonate Process for the Removal of Sulfur Dioxide from Power Plant Stack Gases"
National Technical Information Service
NTIS Number Title ______________________________________ PB 191957 Reduction PB 191958 Regeneration PB 191959 Materials Studies PB 191960 Contactor Development PB 191961 Fly Ash Studies PB 191962 Small Pilot Plant and Test Loop Design PB 191963 Plant Analysis ______________________________________
3. Objectives of this Invention
Some important objectives of this invention are:
a. to combine the advantages of wet scrubbing with the advantages of molten salt chemicals recovery, in order to eliminate some of the handicaps in current processes,
b. to provide a cyclic process employing a reactive aqueous absorption solution which effectively removes oxides of sulfur from gases, regenerates the absorption solution for reuse in the gas cleaning stage, and delivers a sulfur compound readily convertible into elemental sulfur,
c. to provide for economical recovery of the absorption solution,
d. to avoid plugging the absorption stage and other components of the process due to deposition of insoluble components which otherwise may be formed in the absorption step of the process,
e. to minimize the loss of absorption medium due to the formation of sulfur compounds not readily decomposed in the regeneration step for recovery of the original absorption medium,
f. to avoid polluting the atmosphere by the absorption medium,
g. to provide an absorption medium inherently capable of quantitatively removing oxides of sulfur from the gases, and/or
h. to minimize interference with plant operation when the process is installed in existing installations.
These and other objectives will become apparent in the following description of the invention.